1. Field of the Invention
The present invention relates to methods for solidification of waste, apparatus and solidifying materials therefor, and waste forms thereby, especially, to methods for solidification of incombustible miscellaneous solid waste generated from nuclear power stations and calcined ashes generated by calcining treatment of burnable miscellaneous solid waste, apparatus therefor, solidifying materials therefor, and waste forms obtained by the solidifying method.
2. Prior Art
In order to make a final burial treatment to underground of radioactive waste generated from nuclear power plants etc, a treatment for preventing radioactive nuclides contained in the waste from dispersing outside must be performed previously. That is, it is required to add artificial barriers to the waste by solidifying the waste to form stable bodies in exclusive vessels. As for solidifying materials therefor, asphalt, plastics, hydraulic solidifying material such as cements, mortar, and concrete are general, inexpensive, and preferable in characteristics such as mechanical strength etc, and therefore, they have been used widely as solidifying materials for both radioactive and non-radioactive waste. Particularly, the solidifying materials of cement group have been widely used because of their low price and easiness in handling and solidifying operation.
Generally, cements are strongly basic, and are preferable for artificial barriers to seal radioactive nuclides because they solidify the radioactive nuclides such as cobalt-60, nickel-63, and transuranic elements as hydroxide precipitates. Preferable heat resistance and radiation resistance are other advantages of the cements.
As for the radioactive waste generated from nuclear power plants, there are incombustible miscellaneous solid waste such as pipes, valves, metallic waste materials, insulator, concrete chips, and used HEPA filters etc, all of which are generated at operation sites such as periodic inspection etc. Further, there is the calcined ashes generated from calcining treatment of burnable miscellaneous solid waste. In the calcining ashes generated from calcination of burnable waste and the incombustible miscellaneous solid waste, metals such as aluminum and zinc are often included. Particularly, in a gas cooled reactor, aluminum alloy is used as a material for a splitter composing Magnox nuclear fuel, and accordingly, aluminum alloy waste is generated from reprocessing of spent fuel. Therefore, development of technology for solidification of these waste with integral artificial barriers to the above metals is required. However, actually at the present, these waste are packed in drum cans and stored within the site of the power station, and hence, solidification of the waste to stable solidified waste forms and burial treatment to underground are strongly desired.
It has been pointed out that, when the incombustible miscellaneous solid waste and calcined ashes are solidified with basic solidifying material such as cements etc, waste forms having undesired characteristics are often produced. As for the reason, it has been revealed that metallic chips or metallic powder which generate hydrogen gas by reacting with basic components of the cement are slightly included in these waste, and the hydrogen gas generates bubbles and cracks in the waste forms. However, these metals have the same appearance, and it is physically difficult to separate only the undesired metallic chips and metallic powder from the waste.
Prior art to solve the above problems are disclosed in JP-B-2-62200 (1990), JP-A-4-200680 (1992), and JP-A-5751163 (1982).
According to JP-B-2-62200 (1990), when calcined ashes are solidified with cement, a method is disclosed to suppress generation of hydrogen gas after mixing the cement and the calcined ashes by previously mixing the calcined ashes with basic material to complete somewhat the generation of hydrogen gas before the solidification. In the same reference, it is also disclosed that ZnO and PbO in the calcined ashes disturb proceeding of hydration reaction at the solidification of the cement.
However, in accordance with the former method, pretreatment process is required for cement solidification of the calcined ashes, and accordingly, it takes long time for the pre-treatment when a large amount of metals such as aluminum are included in the ashes. On the contrary, the pre-treatment performed on the calcined ashes which do not include the amphoteric metals wastes time and expense. Further, as metals are dissolved in the pre-treatment process, the process causes dispersion of radioactive nuclides into the process solution when the metals are contaminated or radioactive, and consequently, it requires other countermeasure to ensure isolating function as the artificial barriers for the waste form. Furthermore, it is not desirable in view of safety to generate a large amount of combustible hydrogen gas at the pre-treatment process.
The latter description on the hydration reaction discloses only disturbance in the proceeding of hydration reaction by ZnO and PbO at the cement solidification, and does not relate at all to the gist of the present invention which claims that enhancing the hydration reaction makes it possible to suppress generation of hydrogen gas.
The second reference, JP-A-4-200680 (1992), discloses a method for preparing a solid form suppressing generation of hydrogen gas by controlling basic components in the cement at the solidifying process so that the pH of the cement paste, which is a mixture after mixing process of the cement, is reduced to utmost 13 when incombustible miscellaneous solid waste containing aluminum is solidified with the cement.
However, in accordance with the above method, it is possible to reduce generation of hydrogen gas somewhat by controlling pH of the cement paste, but it is impossible to reduce the amount of the hydrogen gas to a level deemed to be approximately zero. Furthermore, as being strongly basic is indispensable condition for stable solidification of cobalt-60 and nickel-63 etc as described previously, excessive lowering of pH reduces the essential function of the cement to solidify the radioactive nuclides stably.
The problem that metals react with basic materials such as cement and generate hydrogen gas is not typical of radioactive waste treatment, but also it is regarded as a problem in the field of general industrial waste. Regarding to a phenomenon that aluminum reacts with basic material to generate hydrogen gas when additives or binders which are equivalent to the solidifying material are added to the calcined ashes of urban waste or industrial waste, JP-A-57-51163 (1982) discloses two methods the one in which a desired waste form can be obtained under a nearly neutral condition as the pH is in a range between 5-9 where aluminum does not generate hydrogen gas and stable, and another method in which a waste form having superior characteristics can be obtained by preparing a waste form after reacting all the contained aluminum previously under a strong basic condition of the pH 10-12.
However, the former method has a difficulty in solidifying medium having the pH other than 5-9 (for instance, hydraulic solidifying materials of cement groups). The latter method has the same problem as JP-B-2-62200 (1990) which similarly treats the contained aluminum previously with an alkaline atmosphere.